Something that applies to many a videogame boss (especially in the case of a Giant Space Flea From Nowhere), Kaiju or Monster of the Week - you come across a creature that looks unlike anything else you've seen or will see in that work's universe, but only once. Bonus Points if it would be particularly hard for an ecosystem to support more than one of it. Creators contemplating for the existence of more of these tends to be a gap in their World Building, but tends to be a case where the MST3K Mantra applies in full force. Examples of Single Specimen Species include: